Mycophenolate mofetil (CellCept), or mycophenolic acid, is a compound that inhibits the creation of guanosine nucleotides, one of the building blocks of DNA and RNA. It is used as an immune suppressant in organ transplantation to prevent rejection.

Mycophenolate reduces the competition between naturally occurring guanine nucleotides and the nucleoside reverse transcriptase inhibitors (NRTIs) that resemble guanine, such as abacavir (Ziagen). This reduced competition enhances the activity of abacavir: in five patients treated abacavir-containing antiretroviral therapy, adding mycophenolate 500mg twice a day reduced viral loads by 1.05 log₁₀ after three to six weeks. Therapy was well tolerated in all cases^[1].

In two other studies, patients taking mycophenolate with antiretroviral therapy experienced similar rates of viral load decline and CD4 cell count increase to those not adding mycophenolate, but with higher rates of side-effects^[2][3]. However, adding mycophenolate before and during an interruption of HIV treatment may prolong the time that a patient can remain off HIV therapy^[4].

Mycophenolate has also been shown to augment the antiretroviral activity of tenofovir (Viread) and ddI (didanosine, Videx / VidexEC) in the test tube. In the case of tenofovir, the addition of mycophenolate was able to suppress replication of the virus with the K65R tenofovir resistance mutation^[5].

A pilot study on the use of mycophenolate in salvage therapy has reported modest effects. Seven patients who had failed to respond to an eight-drug regimen received abacavir with mycophenolate, ddI, amprenavir (Agenerase) and ritonavir (Norvir). After 16 weeks, a transient viral load reduction had been observed, but viral load returned to baseline levels. No decline in CD4 cell count was observed, and T-cell activation declined to normal levels in four out five individuals who remained on treatment for at least 16 weeks^[6].

Further studies are required to assess the efficacy of adding mycophenolate to antiretroviral drug combinations, and its associated side-effects.